Secondary air systems are common in many vehicles and are used to add oxygen to exhaust gases, aiding in final burn off of any undesirable emissions. Generally, secondary air systems operate by pumping fresh air into the exhaust by a simple electrical/mechanical air pump or a special one-way reed valve that utilizes the pulsing of the exhaust from the piston firings to draw the fresh air in. The secondary air, now mixed with exhaust, is injected into the exhaust manifolds, catalytic converters, or both.
In smaller engines, the secondary air system is generally used only at startup to help burn the rich air-to-fuel mixture needed to run a cold engine. Once the catalytic converter is at operational temperature, the secondary air system is shut down as it is no longer needed.